Organizational features of the ipsilateral representation in the cat face motor cortex were investigated by using the technique of intracortical microstimulation (less than 30 microA). In 4 intact animals, 61 efferent zones controlling facial muscles were identified. They were devoted to contralateral muscles (contralateral efferent zones; n = 35), ipsilateral muscles (ipsilateral efferent zones; n = 8) or symmetrical muscles of both sides (bilateral efferent zones; n = 18). Contralateral efferent zones were found within both the rostral part of the coronal gyrus and the lateral bank of the presylvian sulcus, whereas ipsilateral and bilateral efferent zones were exclusively localized to the rostromedial region of the face motor cortex. Movement thresholds proved to be lowest at the contralateral efferent zones and highest at the ipsilateral efferent zones, with intermediate values at the bilateral efferent zones. Latencies of suprathreshold EMG responses evoked from contralateral and ipsilateral efferent zones were the shortest and the longest, respectively; intermediate values were found upon stimulation of sites from bilateral efferent zones. The efferent zones identified in 3 lesioned animals (two cats with contralateral motor cortex ablation and one cat with transection of the rostral two thirds of the corpus callosum) were contralateral (n = 14), bilateral (n = 13), and ipsilateral (n = 9). Mean thresholds of effective sites from bilateral and ipsilateral efferent zones in lesioned preparations were not significantly higher than those in intact animals. This would thus suggest that extracallosal pathways may account for ipsilateral responses
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